The present invention generally relates to liquid crystal display devices and more particularly to a liquid crystal display device operating in a so-called VA (Vertically Aligned) mode in which liquid crystal molecules having a negative dielectric anisotropy are aligned generally perpendicularly to a panel surface of the liquid crystal display device.
Liquid crystal display devices are used as a display device of various information processing apparatuses such as a computer. Liquid crystal display devices, having a compact size and consuming little electric power, are particularly suitable for application in portable information processing apparatuses. On the other hand, use of such liquid crystal display devices also in a fixed-type information processing apparatus such as a desktop-type computer, is also studied.
Conventional liquid crystal display devices generally use a so-called TN (Twisted Nematic)-mode construction in which p-type liquid crystal molecules having a positive dielectric anisotropy are aligned horizontally between a pair of mutually opposing panel substrates, wherein the liquid crystal molecules adjacent to one panel substrate and the liquid crystal molecules adjacent to the other panel substrate are aligned in respective directions crossing with each other perpendicularly.
In such a TN-mode liquid crystal display device, various liquid crystals are already developed, and the liquid crystal display device can be fabricated by a well-established process with low cost.
On the other hand, a TN-mode liquid crystal display device has a drawback in realizing a high contrast representation of images. It should be noted that a TN-mode liquid crystal display device provides a black representation by causing the liquid crystal molecules to align vertically to the principal surface of the panel substrate by applying a driving electric field, while the liquid crystal molecules immediately adjacent to the panel substrate tend to maintain the horizontal alignment even when the driving electric field is applied. Thereby, the birefringence associated with such horizontal liquid crystal molecules allows a passage of light even in the activated state in which the passage of light through the liquid crystal layer should be interrupted completely. Thus, there occurs a leakage of light, or sometimes a coloring of the panel, when an attempt is made in a TN-mode liquid crystal display device to display a white image on a black background (so-called "normally black mode") as is commonly adopted in a CRT display device. Thus, the black representation becomes worse than that of a "normally white mode," in which black images are displayed on a white background, because of the dispersion. This is the reason why conventional TN-mode liquid crystal display devices are operated in the normally white mode.
A VA-mode liquid crystal display device is a liquid crystal display device in which liquid crystal molecules having a negative or positive dielectric anisotropy are confined between a pair of panel substrates in a state that the liquid crystal molecules are aligned in a direction generally perpendicular to the principal surface of the panel substrates in a non-activated state of the liquid crystal display device. Thus, a light passes through a liquid crystal layer in such a liquid crystal display device without changing the polarization plane thereof in the non-activated state of the liquid crystal device, and the light is effectively interrupted by a pair of polarizers disposed at both sides of the liquid crystal layer in a crossed Nicol state. In such a VA-mode liquid crystal display device, therefore, it is possible to achieve a near-ideal black representation in the non-activated state of the liquid crystal display device. In other words, such a VA-mode liquid crystal display device can easily achieve a very high contrast representation not possible by a TN-mode liquid crystal display device.
In an activated state of a VA-mode liquid crystal display device in which a driving electric field is applied to the liquid crystal molecules by as a result of application of a driving voltage exceeding a predetermined threshold voltage, it should be noted that the liquid crystal molecules are aligned generally parallel to the panel substrates, and a substantial rotation is induced in the polarization state of an incident optical beam. Thereby, the liquid crystal molecules thus activated show a 90.degree.--twist between the first panel substrate and the second panel substrate.
FIGS. 1A and 1B show a conventional VA-mode liquid crystal display device 10 respectively in a non-activated state (black representation mode) and an activated state (white representation mode).
Referring to FIGS. 1A and 1B, the liquid crystal display device 10 includes a lower glass substrate 11A and an upper glass substrate 11B disposed so as to face the lower glass substrate 11A, wherein the lower glass substrate 11A carries, on a top surface thereof that faces the upper glass substrate 11B, an electrode pattern 12A and a molecular alignment film 13A such that the molecular alignment film 13A covers the electrode pattern 12A. Further, the upper glass substrate 11B carries, on a bottom surface thereof that faces the lower glass substrate 11A, a transparent electrode pattern 12B and a molecular alignment film 13B such that the molecular alignment film 13B covers the electrode pattern 12B.
Further, the liquid crystal display device 10 includes a liquid crystal layer 14 between the lower and upper substrates 11A and 11B such that the liquid crystal layer 14 is confined in a gap formed between the molecular alignment film 13A on the lower glass substrate 11A and the molecular alignment film 13B on the upper glass substrate 11B, wherein it should be noted that the liquid crystal layer 14 includes liquid crystal molecules 14A each having a negative dielectric anisotropy. Further, the liquid crystal display device 10 includes a first polarizer 15A and a second polarizer called analyzer 15B respectively on the outer sides of the substrates 11A and 11B in a crossed Nicol state.
FIG. 2A shows a pre-tilt angle .theta. of a liquid crystal molecule 14A in a non-activated state of the liquid crystal display device 10.
Referring to FIG. 2A, it can be seen that the liquid crystal molecule 14A forms a tilted angle slightly offset from 90.degree. in the non-activated state of the liquid crystal display device 10. By doing so, the response speed of the liquid crystal display device is improved as compared with the case in which the liquid crystal molecules are directed perpendicularly to the principal surface of the liquid crystal display device 10.
FIGS. 2B and 2C show the molecular alignment film 13A on the substrate 11A and the molecular alignment film 13B on the substrate 11B.
Referring to FIG. 2B, the molecular alignment film 13B is subjected to a rubbing process conducted in a direction rotated by an angle .alpha. in the clockwise direction from a first reference direction ref.sub.1 when viewed from the upward direction of the molecular alignment film 13B. Similarly, the molecular alignment film 13A is subjected to a rubbing process conducted in a direction rotated also by the angle .alpha. in the clockwise direction from a second reference direction ref.sub.2 when viewed from the upward direction of the molecular alignment film 13A, wherein it should be noted that the second reference direction ref.sub.2 is opposite to the first reference direction ref.sub.1. As a result of the rubbing in the upper and lower molecular alignment films 13A and 13B, there is formed a twist angle 2.alpha. in the liquid crystal molecules 14A forming the liquid crystal layer 14.
In the non-activated mode of FIG. 1A, it should be noted that no electric field is applied between the electrode patterns 12A and 12B. Thus, the liquid crystal molecules 14A, having a negative dielectric anisotropy as noted before, are aligned generally perpendicularly to the principal surface of the substrate 11A or 11B as a result of interaction with the molecular alignment film 13A or 13B. As a result of such an alignment of the liquid crystal molecules 14A, the optical beam, incident to the substrate 11A from the downward direction through the first polarizer 15A, experiences little rotation of polarization plane as it propagates through the liquid crystal layer 14. Thus, the optical beam passed through the liquid crystal layer 14 is substantially interrupted by the second polarizer 15B provided on the substrate 11B.
In the activated mode of FIG. 1B, on the other hand, a driving electric field is formed between the electrodes 12A and 12B and the liquid crystal molecules 14A having the negative dielectric anisotropy are aligned generally parallel to the principal surface of the substrate 11A or 11B. As a result of the parallel alignment of the liquid crystal molecules 14A, the optical beam incident to the substrate 11A from the downward direction through the first polarizer 15A experiences a substantial rotation of polarization plane as it propagates through the liquid crystal layer 14. Thus, the optical beam passed through the liquid crystal layer 14 exits from the second polarizer 15B provided on the substrate 11B without being interrupted.
Due to the high-contrast-ratio image representation achieved by a VA-mode liquid crystal display device, which is comparable to that of a CRT display device, it is thought that the VA-mode liquid crystal display device may be used for a display device of so-called desktop type. In order that such a VA-mode liquid crystal display device is used as a practical desktop display device, however, it is further necessary that the liquid display device is capable of providing a large view angle, in addition to large display area and high response speed.
The inventor of the present invention has discovered previously in the VA-mode liquid crystal display device 10 of FIGS. 1A and 1B, in that the view angle of the liquid crystal display device 10 decreases with increasing retardation .DELTA.n.multidot.d of the liquid crystal layer 14, and that the transmittance of the liquid crystal display device 10 in the white representation mode (activated mode), or the brightness of representation, increases with increasing retardation .DELTA.n.multidot.d of the liquid crystal layer 14. See the relationship of FIG. 3.
From the relationship of FIG. 3, it is concluded that the preferable retardation .DELTA.n.multidot.d of the liquid crystal layer 14 falls within the range of about 0.2 .mu.m to about 0.4 .mu.m (0.2 .mu.m&lt;.DELTA.n.multidot.d&lt;0.4 .mu.m). In FIG. 3, it should be noted that the vertical axis at the right represents the transmittance while the vertical axis at the left indicates a threshold view angle in which the contrast ratio just exceeds the value 10.
In the liquid crystal display device 10 of FIGS. 1A and 1B, on the other hand, it was discovered that the voltage holding ratio of the liquid crystal layer 14 decreases with time as indicated in FIG. 4 by a continuous line. In the case of an ordinary TN-mode liquid crystal display device in which the liquid crystal layer has a positive dielectric anisotropy, on the other hand, no such a problem occurs as indicated in FIG. 4 by a broken line. Further, it is noted that the foregoing problem of decrease of the voltage holding ratio becomes conspicuous with increasing magnitude of the dielectric anisotropy .DELTA..epsilon. of the liquid crystal layer. When the voltage holding ratio is reduced as such, the leakage current through the liquid crystal layer is increased and a serious problem of representation such as uneven brightness or color may induced, particularly in the case when the liquid crystal display device uses the art of active matrix driving.
In relation to the use of the liquid crystal molecules having negative dielectric anisotropy for the liquid crystal layer 14, it was further discovered that a d.c. voltage tends to remain in the liquid crystal layer 14 of the VA-mode liquid crystal display device 10. When such a d.c. voltage remains in the liquid crystal layer 14, there tends to occur an afterimage in the representation of the liquid crystal display device 10.
It should be noted that the foregoing problem of reduction of the voltage holding ratio and the lasting of the residual d.c. voltage can be avoided by decreasing the magnitude of the negative dielectric anisotropy .DELTA..epsilon.. However, such a decrease of the magnitude or absolute value of .DELTA..epsilon. invites a problem of the liquid crystal molecules less responding to the applied voltage and a corresponding decrease of retardation .DELTA.n.multidot.d in the liquid crystal layer. Thus, there arises a problem in that the liquid crystal display device can no longer satisfy the condition of view angle characteristics and transmittance explained with reference to FIG. 3. Further, such a decrease of magnitude of the dielectric anisotropy .DELTA..epsilon. tends to invite an increase of on/off voltage of the liquid crystal layer 14, while such an increase of the on/off voltage of the liquid crystal layer 14 necessitates a specially designed drive circuit for driving the liquid crystal layer 14.